The head of the earwig Forficula auricularia (Dermaptera) and its evolutionary implications

Abstract

The external and internal head morphology including the musculature of the common earwig Forficula auricularia is described in detail. We specified and corrected previous descriptions and provided a detailed documentation. The head of Forficula is characterized by prognathism, generalized mandibles with a mesal cutting edge distally of the mola and its drop-shaped appearance. We added the following new apomorphies for Dermaptera to the various previously reported ones: (I) coronal and frontal cleavage lines with corresponding strong internal strengthening ridges; (II) the division of the praementum into a basal and a distal sclerite; (III) the presence of a bumpulus at the tip of the paraglossa; (IV) the presence of large distal palpilla on the terminal maxillary and labial palpomeres and (V) the origin of M. tentoriohypopharyngealis on the submentum. Another potential apomorphy are the lateral lobes on the distal hypopharynx which are most likely not homologous to the superlingua of apterygote insect. Other characters such as the prominent ball-and-socket joint between scapus and pedicellus or the unique antennal heart are poorly studied within Dermaptera and therefore cannot be phylogenetically polarized. In contrast to the various ordinarial apomorphies, the dermapteran head exhibits only one potential synapomorphy with other polyneopteran orders: the absence of the linguactual tendon and the associated muscle that is shared with Plecoptera and/or Zoraptera. Our results show that additional studies of the presumingly basal splits of Dermaptera are required to understand the head evolution of the group.